1. Field of the Invention
The present invention relates to a method for controlling the air-fuel ratio in an internal combustion engine, more particularly to such a method utilized in an automobile engine having an electronically controlled fuel injection device.
2. Description of the Related Art
In one known type of apparatus for controlling the air-fuel ratio of an internal combustion engine, there is provided a means for generating a fundamental fuel injection signal representing the amount of fuel to be injected to the engine in a normal state based on predetermined operational parameters of the engine, e.g., engine temperature; means for detecting a transient operating condition of the engine; means, responsive to the measured engine temperature and the detected transient operating condition of the engine, for generating a correction promotion signal which has an initial value equal to a first value determined by the engine temperature and determined by the detected transient condition of the engine and which is increased by a factor which changes to unity (1), at a speed determined by the engine temperature; and means for supplying fuel to the engine based on the fundamental fuel injection signal and the correction signal, thereby supplying fuel to the engine in either the normal state or the transient state of the engine. This apparatus provides a fuel supply system which allows optimum operation of the engine by constant maintenance of the optimum air-fuel ratio in either the normal state or the transient state of the engine (e.g., Japanese Unexamined Patent Publication (Kokai), No. 56-6034.)
In the apparatus of the type described above, however, no consideration is given to changes in characteristics caused by changes in engine performance over time, e.g., buildup of deposits at a valve clearance or at an injector nozzle in an electronic fuel injection (EFI) system, buildup of deposits (deposits of fine carbon particles from lubricant components or fuel combustion products) at the rear surface of a cylinder intake valve or the like, or changes in the volatility of gasoline due to variations in the properties of the gasoline. The apparatus described above does not have means for compensating for deviations or variations from an optimum value of the air-fuel ratio during acceleration due to changes in characteristics of the engine or changes in the properties of gasoline. If gasoline of a low volatility is used or the operating characteristics of the engine change with time, the drivability of the engine is apt to be deteriorated causing uneven acceleration due to a lean fuel mixture during acceleration. Conversely, when gasoline of a high volatility is used, the air-fuel ratio becomes rich, resulting in a decrease in mileage or poor emission.
In the normal state, clogging of an injector can be corrected by feedback of an output from an air-fuel sensor. However, since no correction means is incorporated in consideration of acceleration, similar problems are encountered. Furthermore, similar problems are also encountered due to variations in the manufacture of engines and air-flow meters or changes over time thereof.